1. Field of the Invention
The present invention relates to a plugged honeycomb structure. The present invention more particularly relates to a plugged honeycomb structure which is capable of suppressing an increase of a pressure loss and which is useful as a trapping filter for an exhaust gas, especially a diesel particulate filter (DPF) to trap particulate matter (particulate) and the like in an exhaust gas of a diesel engine.
2. Description of the Related Art
A honeycomb structure having porous partition walls is broadly used in a filter, a catalyst carrier or the like, and is broadly used especially as a catalyst carrier, a filter or the like for purifying or treating an exhaust gas of an internal combustion engine such as a gasoline engine or a diesel engine, or a combustion device.
The exhaust gas discharged from an internal combustion engine such as the diesel engine includes a large amount of particulate matter (particulate) containing, as a main component, carbon which is a cause for environmental contamination. Therefore, a filter for trapping the particulate matter (particulate) such as a diesel particulate filter (DPF) is sometimes mounted in an exhaust system of the engine.
In general, examples of the honeycomb structure to be used for such a purpose include, as shown in FIGS. 6 and 7, a honeycomb structure 12 having a plurality of cells (circulation holes) 19 partitioned by porous partition walls 17 and extending through the structure in an axial direction; and a plugged honeycomb structure 11 in which one end portion of each predetermined cell 19a is plugged with a plugging portion 21 constituted of a plugging material filled in this cell 19 and in which an end portion of each remaining cell 19b on a side opposite to the plugged end portion of the predetermined cell 19a is plugged with the plugging portion 21 (see, e.g., Patent Document 1).
The exhaust gas flows into a filter constituted of such a honeycomb structure (the plugged honeycomb structure 11) from one end surface 13, and flows out of the other end surface 15 after the particulate matter included in the gas and the like are removed.
Specifically, the exhaust gas flows into the cells 19b whose end portions in the inflow-side end surface 13 of this filter are not plugged and whose end portions in the outflow-side end surface 15 are plugged, flows through the porous partition walls 17, moves to the cells 19a whose end portions in the inflow-side end surface 13 are plugged and whose end portions in the outflow-side end surface 15 are not plugged, and is discharged from the cells 19a. 
Moreover, in this case, the partition walls 17 constitute filtering layers, and the particulates in the exhaust gas are trapped by the partition walls 17 and deposited on the partition walls 17.
In recent years, increase of porosities of the partition walls of the honeycomb structure and thinning of the walls have advanced in order to reduce a pressure loss at a time when the exhaust gas is treated with the honeycomb structure and treat the exhaust gas more efficiently.
Moreover, as a method of avoiding a rapid increase of the pressure loss due to the deposited particulate matter, there is proposed a plugged honeycomb structure in which a plugging member is provided with a protruding portion protruding in a shape tapered from the end surface of the cell toward an upstream side (see, e.g., Patent Document 2).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-269585
[Patent Document 2] Japanese Patent Application Laid-Open No. 2002-309922
However, even in such a conventional plugged honeycomb structure, it cannot be said that an effect of suppressing the increase of the pressure loss is sufficiently exhibited. There has also been a problem concerning improvements of a treating property, an impact resistance and the like.
Moreover, in order to reduce a fuel consumption of the engine and treat the exhaust gas efficiently, it is necessary to reduce the pressure loss of the plugged honeycomb structure in an initial state, that is, in a state in which any particulates or the like is not attached, and there is demanded development of the plugged honeycomb structure in which the pressure loss in the initial state is reduced in this manner.